This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-208724, filed Jul. 10, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to an active matrix substrate and a method of manufacturing active matrix substrates
2. Description of the Related Art
Liquid-crystal displays (LCDs) have been widely used as display terminals for mobile information units, such as notebook personal computers, televisions, mobile phones, or mobile information terminals (or PDAs). For example, an active matrix LCD is formed as follows: thin film transistors (TFTs), which use amorphous silicon or polycrystalline silicon as an active layer, are formed in a matrix on a glass substrate and secured with an approximately 5-xcexcm gap between the TFTs and an opposite glass substrate, the gap being filled with liquid crystal, thereby completing an active matrix LCD. This type of active matrix LCD is used as a thin display unit that provides high-quality, full-color display.
On the other hand, there have been demands that LCDs should consume less electric power, have a larger number of pixels, be larger in size, weigh less, help decrease manufacturing costs, assure high-quality display, etc.
Active elements, such as TFTs, are formed by repeating the following processes: electrodes, an insulating layer, etc. are formed on a glass substrate by vacuum processes, including CVD and sputtering techniques, and then are subjected to photolithography and dry etching or wet etching, thereby forming a pattern. Therefore, to obtain a large display unit, it is necessary to make the apparatus for vacuum processes larger, resulting in higher manufacturing costs. Since the percentage of the area of the display unit taken up by the active elements is small, it is wasteful to use a large apparatus for vacuum processes.
To make a display unit lighter, the formation of TFTs on a plastic substrate or a film substrate has been studied. Forming TFTs on those substrates requires the process temperature to be lowered. However, it is conceivable that a drop in the process temperature will degrade the TFT performance and therefore impose a limitation on the picture quality, the number of pixels, etc. Moreover, since those substrates have high thermal expansion coefficients and are deformed plastically at low temperature, a higher definition design is expected to be impossible, which leads to a decline in the quality of display.
A method of solving those problems has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-7340. In the method, active elements, such as TFTs, are formed on an element formation substrate made of glass, silicon, or the like and then selectively transferred to another display substrate (or a final substrate) made of plastic, film, or the like. Thereafter, the active elements are interconnected.
FIG. 1 shows one step in the method of transferring and forming active elements disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-7340. In this method, active elements 2701, such as TFTs, are formed on an element formation substrate (not shown) and then transferred to an intermediate substrate 2702. The active elements 2701 on the intermediate substrate 2702 are further transferred to a final substrate 2703.
In this method, the active elements 2701 and the intermediate substrate 2702 are bonded together via a temporary adhesion layer 2704. The temporary adhesion layer 2704 is made of a material that has adhesion and lowers in adhesion when being illuminated by light or heated. As shown in FIG. 1, adhesion layers 2705 are formed beforehand in regions on the final substrate 2703 where the active elements 2701 are to be transferred. The active elements 2701 are transferred in such a manner that light or heat is projected via a mask 276 onto only the active elements to be transferred, while the active elements 2071 are being pressed against the adhesion layers 2705.
In the method disclosed Jpn. Pat. Appln. KOKAI Publication No. 2001-7340, active elements have been formed on an element formation substrate with a high density and then selectively transferred to a final substrate, thereby improving the production efficiency. In this method, however, since the positions of the active elements are not determined on the surface of the final substrate, the accuracy of the alignment of the intermediate substrate with the final substrate determines the accuracy of the positions of the active elements. Therefore, when the final substrate is produced by a plurality of transfers, the positions of the active elements shift, resulting in variations in the shift. When interconnections are made after the process of transferring the active elements, a shift in the position causes variations in the parasitic capacitance between the active elements, inter-connections, and electrodes, resulting in a decline in the quality of display.
Furthermore, to increase the position accuracy in transferring the active elements, there is a method of making tapered hollows in a final substrate and transferring tapered active elements to the final substrate.
In this method, since both of the active elements and the final substrate have been tapered, the active elements fit into the tapered hollows in the final substrate, achieving the transfer with high position accuracy. In this transfer, however, since the active elements are pushed into the hollows in the final substrate, thereby carrying out transfer, this makes it difficult to selectively transfer densely formed active elements to the final substrate.
As described above, in forming an active matrix substrate, it was impossible to selectively transfer active elements with high position accuracy by a conventional method of transferring active elements. Therefore, there has been a need to realize an active matrix substrate which enables active elements to be selectively transferred with high position accuracy and a method of manufacturing such active matrix substrates.
According to a first aspect of the present invention, there is provided an active matrix substrate comprising: a substrate; a position control member provided on the substrate and having an opening at a central position thereof to expose a top surface of the substrate the opening being surrounded by a sidewall of the position control member, the position control member having an inner side face which inclines at a specific angle with respect to the substrate; an active element having a fitting member abutting the bottom of said active element, said active element being positioned on the position control member and configured to engage with the inner side face of the position control member and an outer side face of the fitting member has at least a part that inclines at substantially the same angle as the specific angle of the inner side face of the position control member with respect to the substrate; and an adhesion section which includes an adhesive that bonds the active element to the position control member and whose wettability with the position control member is lower than that of the adhesive with the substrate.
According to a second aspect of the present invention, there is provided an active matrix substrate comprising: a substrate; a position control member provided on a top surface of the substrate and has a concave part that is made up of an inner side face inclining at a specific angle with respect to the substrate and a bottom part connecting to the inner side face; an active element provided on the position control member, a fitting member abutting a bottom surface of the active element so as to engage with the concave part of the position control member and an outer side face of the fitting member has at least a part that inclines at substantially the same angle as the specific angle of the inner side face of the position control member; and an adhesion section which includes an adhesive that bonds the active element to the substrate or the position control member, with a contact angle of the adhesive to the position control member being 70xc2x0 or more.